Gravitational field strength to distance graph

Question

Can someone please explain why the graph is linear from the start to the surface?

Answer 1

From Gauss's law for gravity we have:

$$\frac{1}{4\pi}\int\mathbf{g}\cdot d\mathbf{a}=-G M_{enc}$$

$$\frac1{4\pi}g(r)\cdot4\pi r^2=-G\rho\cdot\frac 43\pi r^3$$

$$\mathbf{g}(r)=-\frac43\pi\rho G r \mathbf{\hat r}$$